The present invention relates to ceramic investment shell molds for casting molten metals and alloys and, more particularly, to ceramic shell molds that are fiber reinforced to improve mold strength at high casting temperatures.
Both the investment casting process and the lost wax shell mold building process are well known, for example, as is apparent from the Operhall U.S. Pat. Nos. 3,196,506 and 2,961,751. The lost wax shell-mold building process involves repeatedly dipping a wax or other fugitive pattern of the article to be cast in ceramic slurry to provide a ceramic slurry layer, draining excess slurry, stuccoing the slurry with coarse ceramic particles to provide a stucco layer on the slurry layer, and drying the layers to build up a shell mold of desired wall thickness on the pattern. The green shell mold/pattern assembly then is subjected to a pattern removal operation to selectively remove the pattern from the shell mold. A commonly used wax pattern removal technique involves flash dewaxing where the green shell mold/pattern assembly is placed in an oven at elevated temperature to rapidly melt the wax pattern from the green shell mold. Following pattern removal, the green shell mold is fired at elevated temperature to develop mold strength for casting of molten metal or alloy therein.
Conventional lost wax ceramic shell molds can be prone to mold cracking or splitting during the pattern removal operation described above.
Attempts have been made to raise the capability of ceramic shell molds in the DS casting of superalloy components. For example, U.S. Reissue Pat. No. 34,702 describes in one illustrative embodiment wrapping alumina-based or mullite-based reinforcement fiber in a continuous spiral about an intermediate mold wall thickness as it is being built-up. U.S. Pat. No. 6,364,000 discloses in one illustrative embodiment positioning one or more continuous carbon-based reinforcement fibers in a ceramic shell mold wall to this end.
The present invention involves a method of making a ceramic shell mold comprising repeatedly coating a fugitive pattern of an article to be cast with a ceramic slurry layer and applying on the ceramic slurry layer a refractory stucco to form a plurality of ceramic slurry layers and stucco layers on the pattern wherein at least one of the stucco layers is formed by applying discontinuous stucco fibers followed by applying a granular stucco particles on the discontinuous stucco fibers.
In a preferred embodiment of the invention, the granular stucco particles are applied on randomly oriented discontinuous stucco fibers to pack the discontinuous stucco fibers down on the slurry layer underlying the discontinuous fibers. The granular stucco particles preferably are applied on the discontinuous stucco fibers while the underlying slurry layer is still wet such that a majority of the packed down discontinuous stucco fibers stick to the slurry layer. The granular stucco particles preferably are applied on the randomly oriented discontinuous stucco fibers to form a stucco layer comprising a mat of the discontinuous stucco fibers and the granular stucco on and in the mat.
In an illustrative embodiment offered to illustrate but not limit the invention, the granular stucco particles are applied by raining the granular stucco particles by gravity down on the discontinuous stucco fibers.
The present invention also provides a ceramic shell mold wherein at least one of the stucco layers comprises the discontinuous stucco fibers and the granular stucco particles.
Shell molds pursuant to the invention are advantageous to resist mold splitting during the pattern removal operation.
The present invention will become more readily apparent from the following detailed description.